Hi your son probably had a sebacous cyst
in his armpit which was infected .sebacous glands are sebum producing glands in the dermis
the deep layer of skin and these glands open on the surface of the skin through a pore if that pore is blocked due to some reason the secretion accumulate behind and produce a lump it may or may not be infected.
You should keep it clean by washing it dry it and use antiseptic like pyodine then apply antibiotic like mepirocin or Fucidic acid cream'you can give some NSAIDS like iboprofen or a course of oral antibiotic if it is infected .
IF recurrence occur then you can have it excised.